Constructing a smooth dense SiO2 layer by adjusting the hydrolysis rate to promote the magnetic properties of soft magnetic composites

Abstract

It remains a difficult task to generate a smooth dense SiO₂ insulating layer for soft magnetic composites (SMCs). In this work, the effects of adjusting TEOS feeding frequency on hydrolysis/condensation rates and SiO₂ layer nanostructures were investigated. The results showed that the lowest feeding frequency (1 batch in 4 h) led to rapid hydrolysis of TEOS and condensation of silanol monomers, resulting in adhesion of large SiO₂ spheres on the bottom layer. When feeding frequency increased (2 or 4 batches in 4 h), TEOS hydrolysis rate and silanol condensation rate declined, causing adhered SiO₂ spheres to gradually decrease in size until a dense, smooth SiO₂ layer was formed. The Si content and powder flowability of FeNi@SiO₂ also increased with higher feeding frequency. The real permeability of FeNi@SiO₂ SMCs (1–10 MHz) gradually decreased and the anti-saturation performance basically improved with the increase in feeding frequency and TEOS dosage, owing to the increase in nonmagnetic substances revealed by the reduced core density. With regard to magnetic loss, FeNi@SiO₂-4-8 has the lowest magnetic loss tangents at a high frequency (>2 MHz) and the highest core loss at a low frequency (<2 MHz), owing to high resistivity and low permeability caused by the smooth dense SiO₂ layer. In addition, permeability variation rates of all FeNi@SiO₂ SMCs are below 5% after 6 weeks at 85 °C and 85% RH. This work provides an in-depth understanding of the formation mechanism of smooth dense SiO₂ coatings and a viable strategy for promoting the overall performance of SMCs.